This invention relates to the manufacture of flexible plastic film, such as polyethylene film, for use as bags, sheets, and the like.
Polyethylene film is typically created using a tube extrusion process, in which a tube of the soft plastic is extruded through an annular die, and is then inflated and expanded until the tube wall is a desired thickness. The film is typically made anywhere in the range of about 0.3 mils to several mils, depending upon the end use. After the tube of material is extruded, it is drawn vertically upward, typically a distance of forty feet or more. The air within the tube expands the tube outward until the plastic sets. On its journey upwards, the extruded tube has to be flattened so that it may pass between rollers. The flattening is accomplished by a collapsing assembly, in which two facing arrangements of collapsing boards urge the tubular extrusion to a flattened state by the time it reaches the rollers.
Typically, this arrangement consists of a pair of facing collapsing frames, each of which has a number of collapsing boards which are horizontal and arranged in parallel, one above the other on the frame. There are typically sixty to ninety boards on a side, the boards being from four to twelve feet in length. These collapsing boards are formed of high quality hardwood, usually a close grained maple, which is extremely expensive. The boards have a curved or arcuate face against which the plastic film passes. The polyethylene film is quite abrasive to the wood. Even very high quality hardwood as typically used will tend to wear out somewhat unevenly. These boards thus have to be replaced periodically and this is an expensive and time consuming procedure. To replace the collapsing boards on the frame, the boards and any associated hardware have to be unscrewed, so that the entire board can be replaced. The boards are typically four to twenty feet in length, and can be quite heavy and unwieldy for a workman to manage. There is considerable down time involved with collapsing board replacement, as it now takes about three full days to replace all the collapsing boards.